1. Field of the Invention
This invention relates generally to protection modules for telephone lines and data communication systems, and more particularly relates to circuit arrangements for limiting the frequency spectrum of a transmitted signal on a balanced circuit for the purpose of controlling interference.
2. Description of the Prior Art
Telephone systems are designed with central switching stations to which telephone lines are connected. The central switching stations route the telephone calls placed by telephone service subscribers. To route the telephone calls, it is necessary to attach very expensive electronic equipment (e.g., switching networks) to the telephone lines. Energy surges, due to lightning, for example, on the telephone lines can damage the electronic equipment. In order to protect the electronic equipment, energy surge protection circuits are typically placed between the electronic equipment and the telephone lines.
FIG. 1 illustrates a functional block diagram of a telephone system with an energy surge protection circuit. Prior art energy surge protection circuits have included fuses, semiconductor devices, heat coils, gas tubes, or combinations thereof. In FIG. 1, a protection circuit 2 is connected to transmission lines of a telephone or communication system including a tip input line 4, a tip output line 6, a ring input line 8, and a ring output line 10.
A schematic diagram of the conventional prior art protection circuit shown in block form in FIG. 1 is illustrated in detail in FIG. 3. The circuit of FIG. 3 includes semiconductor devices 14, 16 and heat coils 18, 20 connected to the tip input line 4, the ring input line 8, and to ground. Semiconductor device 14 is connected between tip input line 4 and ground. Semiconductor device 16 is connected between ring input line 8 and ground. Heat coil 18 is connected between tip input line 4 and tip output line 6. Heat coil 20 is connected between ring input line 8 and ring output line 10. The semiconductor devices 14, 16 are typically avalanche type devices (e.g., diacs or transorbs). In the event there is an energy surge on the tip line or ring line, the semiconductor device begins sinking current and dissipates the energy surge by, effectively, shorting the tip line or ring line to ground. If, however, the semiconductor device malfunctions or the energy surge is of long duration, the heat coil begins to heat up as current passes through it. After the heat coil has risen to a predetermined temperature, the heat coil triggers a switch which shorts the respective tip line or ring line to ground, thereby further dissipating the energy surge.
Prior art protection circuits are concerned only with protecting electronic equipment from energy surges which occurred at a remote location and propagated through the telephone or data communication line. The prior art energy surge protection circuits are not capable of controlling the frequency signal that is transmitted over the telephone or data communication lines. As high speed transmissions become more popular and equipment that can transmit at frequencies above the intended operating frequency of a telephone or communication system become more prevalent, a problem has developed. Telephone service subscribers often attach high speed equipment to normal telephone lines. The high speed equipment operates at frequencies exceeding the intended operating frequency of the telephone or communication systems. This results in interference on the telephone line and reduced quality of performance. In a multi-channel system, the result is crosstalk. Crosstalk is interference on one channel caused by signals present on other channels.
Conventional circuits for controlling frequencies on a transmission line include various types of filters which are comprised of capacitors, inductors, semiconductor devices, or some combination thereof. These prior art circuits, however, are unbalanced (i.e., ground is used as a signal reference). Telephone lines must be balanced (i.e., the signal must be placed between the tip line and ring line, not the tip line and ground or the ring line and ground). Balanced lines have the advantage of providing improved noise immunity over unbalanced lines which are vulnerable to noise and transients.
None of the prior art circuits overcomes the problem of interference and crosstalk caused by a telephone service subscriber exceeding the intended operating frequency of the telephone system.
It is an object of the present invention to provide a spectrum limiting circuit which prevents telephone subscribers from exceeding a telephone or communication system""s operating parameters.
It is another object of the present invention to provide a balanced spectrum limiting circuit for telephone and data communication systems.
It is a further object of the present invention to provide an apparatus which protects against energy surges on telephone and communication lines and limits the frequency spectrum of signals carried by the lines.
It is an even further object of the present invention to provide a modular design for a telephone and communication line protection module so that a spectrum limiting circuit housed therein may be easily changed for different bandwidths or frequencies of operation.
In accordance with one form of the present invention, a balanced spectrum limiter for use in a telephone or data communication system includes a first inductor wound around a first core and connected to the telephone tip line, a second inductor wound around the first core and connected to the telephone ring line, and a capacitor connected between the tip line and the ring line. Preferably, the first core is in the form of a toroid. The balanced spectrum limiter may also include a protection circuit having a first semiconductor device connected between the tip line and ground, a second semiconductor device connected between the ring line and ground, a first shunting device, such as a heat coil, connected to the tip line, the first inductor, and ground, and a second shunting device, which may also be a heat coil, connected to the ring line, the second inductor, and ground. The first and second shunting devices provide failsafe protection to the electronic equipment to which the transmission lines are connected. Should the duration of the energy surge be longer than what the semiconductor devices can dissipate without their failing, then the shunting devices heat up to a point where they short the tip or ring line to ground, thus dissipating the energy surge before it reaches and damages the electronic equipment.
A preferred embodiment of the present invention further includes a third inductor connected between the first inductor and a tip output line and wound around a second core, a fourth inductor connected between the second inductor and a ring output line and wound around the second core, and a second capacitor connected between the tip output line and the ring output line. Preferably, the second core has a closed magnetic path and an air gap, such as a pot core.
Preferably, the balanced spectrum limiter is removably connected to the protection circuit and can be easily interchanged with other limiter circuits having different bandwidths or center frequencies. The modular balanced spectrum limiter comprises a filter circuit removably connected in series to an energy surge protection circuit. The filter circuit and the energy surge protection circuit are contained in an openable housing. The openable housing is arranged to be received by a connection block thereby placing the filter circuit and the energy surge protection circuit in operable contact with a switching station network. The openable housing includes a body with a top opening and a bottom opening. A base is arranged to be removably attached to the body and close the bottom opening. A top cover is arranged to be removably attached to the body and close the top opening. A cap is arranged to be removably attached to the top cover. The cap contains the balanced spectrum limiting circuit having one or more filters.
The present invention also includes a method of frequency filtering a balanced signal including the steps of winding a twisted pair of conductors around a core thereby forming a pair of inductors, connecting a capacitor between the pair of inductors, receiving a balanced signal on the twisted pair of conductors, frequency filtering the balanced signal, and outputting the filtered balanced signal. The filtered balanced signal may then be put through a second filter to achieve greater frequency filtering.
These and other objects, features, and advantages of the present invention will be apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.